EP0922943A2 - Radiation detecting device and radiation detecting method - Google Patents

Abstract

A radiation detecting device having a wavelengthconverter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, thedetecting device comprises a means for turning on theTFT to be turned on first among the TFTs for transferafter the delay of at least (n × τ₁), wherein τ₁ is atime constant of a characteristic of the wavelengthconverter, and n is ln(SN) in which SN is a desiredsignal to noise ratio, after the irradiation isstopped, thereby transferring a signal stored in itscorresponding pixel.

Description

BACKGROUND OF THE INVENTIONField of the Invention

The present invention relates to a radiationdetecting device and a radiation detecting method, andmore particularly to a radiation detecting device and aradiation detecting method which are suitable for usein detecting information such as images by convertingthe wavelength of radiation including X-rays by meansof a wavelength converter typified by a scintillator(or phosphor) into a wavelength in a wavelength regiondetectable by a sensor element.

Related Background Art

When radiation such as an X-ray is detecteddirectly by a photosensor in radiation diagnosticapparatus and X-ray photographic apparatus making useof an X-ray or the like, the efficiency of such anapparatus becomes poor because there is no photosensorhaving high sensitivity to the radiation. It istherefore considered to use a scintillator capable ofconverting the radiation into visible light and aphotosensor in combination.

As the characteristics of the scintillator, thereare characteristics called the afterglowcharacteristics of a luminescent screen. It is indicated that the light emission of a scintillator,which is attendant on radiation exposure, is caused andattenuated in a certain functional relation asillustrated in Fig. 1, and a slow component has a timeconstant as long as several hundreds milliseconds. Inorder to correct the attenuation of afterglow as acountermeasure thereof, in U.S. Patent No. 5,331,682 byway of example, a great number of signal samples aredetected to calculate out a compensation value bycomplicated calculation, and the compensation value issubtracted from the signals. In addition, for thiscalculation, delay is caused until the initialattenuation component can be neglected.

On the other hand, it is proposed in, forexample, U.S. Patent No. 5,262,649 to use a photosensorcomposed of thin film semiconductors in combinationwith a scintillator for X-ray photographic apparatusand radiation diagnostic apparatus making use of an X-rayor the like. In this publication, a relationshipamong the time constant according to the sensorcomposed of the thin film semiconductor, and atransistor, the read rate of the apparatus, and an S/N(signal/noise) ratio is described. In U.S. Patent No.5,262,649, there are introduced a reading method of afluoroscopic mode in which an X-ray is continuouslyemitted, and a photographic mode in which an X-ray isemitted only for a short period of time, and all the sensors store signals at the same time.

However, in order to detect a great number ofsignals to calculate out a compensation value, and tomake the calculation that the compensation value issubtracted from the signals as described in, forexample, U.S. Patent No. 5,331,682, expensive signalprocessing circuit and arithmetic unit are required.In addition, since delay is caused until the initialattenuation component can be neglected, the fetch ofsignals from a detector requires to wait by the delaytime.

In U.S. Patent No. 5,262,649, the fluoroscopicmode in which an X-ray is continuously emitted, and thephotographic mode in which an X-ray is emitted only fora short period of time are introduced. In thephotographic mode, no time constant of the lightemission and attenuation of the scintillator isconsidered. Therefore, when reading is started in amoment after the irradiation with the X-ray iscompleted, due to the time constant of the lightattenuation of the scintillator, a signal is read outin the initial line of the reading while a dark currentis high, and a signal with a dark current componentintegrated is read out in a reading line on the lastside. Therefore, the dark current mixed into thesignal due to the delayed attenuation characteristicsof the scintillator greatly varies according to the reading order of the line.

In U.S. Patent No. 5,262,649, there is introducedan X-ray diagnostic apparatus or radiation therapeuticapparatus using a large screen sensor panel comprisedof sensors composed of a-Si:H (amorphous siliconhydride) and thin film transistors, and a relationshipamong a time constant obtained by multiplying thecapacity of the sensor by the ON resistance of the thinfilm transistor, an S/N ratio and a frame frequency isderived, which is required as a real-time image sensor.However, this relationship is on the assumption that anX-ray is continuously emitted, and such attenuationcharacteristics of the scintillator as described aboveare not referred to. This publication does also notrefer to the design of reading when an X-ray isintermittently emitted.

The attenuation characteristics of thescintillator do not become a considerable problem inthe case of the photographic mode or the like becausethere is sufficient time. In the case of a full movingimage having many frames as in the diagnosis of thecirculatory organ system, however, it is consideredthat the residual component of light may exert aninfluence as noise.

In such a case, however, it is not proposed tomake a design by combining the attenuationcharacteristics of the scintillator with the reading characteristics of the time constant composed of thecapacity of the sensor and the ON resistor of the thinfilm transistor in the sensor panel in such a case.

SUMMARY OF THE INVENTION

It is an object of the present invention to readout signals of a desired S/N ratio, which are reducedin noise and narrowed in scattering, by adopting areading method taking the attenuation characteristicsof a scintillator into consideration in a radiationdetecting device for a radiation diagnostic apparatusor the like which is capable of reducing an exposeddose by intermittent exposure to radiation such as X-rays.

Another object of the present invention is toderive a relationship for obtaining an optimum signalto noise (S/N) ratio taking the attenuationcharacteristics of a scintillator into consideration inthe inspection, diagnosis and therapy with radiationcontinuously emitted.

The above objects can be achieved by the presentinvention described below.

According to the present invention, there is thusprovided a radiation detecting device having awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixel comprises a sensor element for convertig the light intoan electric signal and a thin film transistor (TFT) fortransfer connected to the sensor element tosuccessively transfer a signal from the pixel, thedetecting device comprising:
   a means for turning on the TFT to be turned onamong of the TFTs for transfer after the delay of atleast (n x τ₁), wherein τ₁ is a time constant of acharacteristic of the wavelength converter, and n isln(SN) in which SN is a desired signal to noise ratio,after the irradiation is stopped, thereby transferringa signal stored in its corresponding pixel.

According to the present invention, there is alsoprovided a radiation detecting device comprising awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, whereinthe detecting device satisfies the following relationalexpressions:(α × τ1 + β × τ2) ≤ 1/FPS;andSN = exp(α + β)wherein SN is the desired signal to noise ratio of thewhole device, FPS is the number of frames per second upon the reading of the radiation detecting device, ora reciprocal of the time required for a reading; τ₁ is atime constant of build up and attenuation of thewavelength converter; τ₂ is a time constant obtained bymultiplying the capacity of the sensor element by theON resistance of the TFT for transfer; α is a multipleof [(storage time of the light signal in the sensorelement)/τ₁], or ln(SN₁) in which SN₁ is a signal tonoise ratio required of the wavelength converter; and βis a multiple of a time constant of the time the TFTfor transfer is turned on, or ln(SN₂) in which SN₂ is asignal to noise ratio required of the TFT fortransferring the signal stored in the capacitor of thesensor element.

According to the present invention, there isfurther provided a radiation detecting method using awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, themethod comprising:
   turning on the TFT to be turned on first amongthe TFTs for transfer after the delay of at least (n × τ₁), wherein τ₁ is a time constant of acharacteristic of the wavelength converter, and n isln(SN) in which SN is a desired signal to noise ratio,after the irradiation is stopped, thereby transferringa signal stored in its corresponding pixel.

According to the present invention, there isstill further provided a radiation detecting methodusing a wavelength converter for converting radiationinto photoelectrically convertible light and aplurality of pixels arranged in the form of a matrixwhich pixel comprises a sensor element for convertingthe light into an electric signal and a thin filmtransistor (TFT) for transfer connected to the sensorelement to successively transfer a signal from thepixel, the method comprising satisfying the followingrelational expressions:(α × τ1 + β × τ2) ≤ 1/FPS; andSN = exp(α + β)wherein SN is the desired signal to noise ratio of thewhole device; FPS is the number of frames per secondupon the reading of a radiation detecting device, or areciprocal of the time required for a reading; τ₁ is atime constant of build up and attenuation of thewavelength converter; τ₂ is a time constant obtained bymultiplying the capacity of the sensor element by theON resistance of the TFT for transfer; α is a multiple of [(storage time of the light signal in the sensorelement)/τ₁], or ln(SN₁) in which SN₁ is a signal tonoise ratio required of the wavelength converter; and βis a multiple of a time constant of the time the TFTfor transfer is turned on, or ln(SN₂) in which SN₂ is asignal to noise ratio required of the TFT fortransferring the signal stored in the capacitor of thesensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 diagrammatically illustrates an example ofthe afterglow characteristics of a scintillator.

Fig. 2 is a schematic circuit diagramillustrating an example of the schematic constructionof a radiation detecting device.

Fig. 3A is a schematic plan view illustrating anexample of a photoelectric conversion circuit part.

Fig. 3B is a schematic cross-sectional view takenon line 3B-3B of Fig. 3A.

Fig. 4 is a schematic circuit diagramillustrating an example of a photoelectric conversionpart.

Fig. 5 is a timing chart illustrating an exampleof the reading operation timing of a radiationdetecting device.

Fig. 6 diagrammatically illustrates an example ofthe signal to noise ratio of sensor output.

Figs. 7A, 7B, 7C, 7D and 7E are timing chartsillustrating an example of reading operation timingupon reading out a moving image.

Fig. 8 diagrammatically illustrates an example ofthe relationship between the transfer time of TFT andthe quantity of signals transferred.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention achieves the reading ofinformation with higher precision and stability bytaking the time characteristics related to thewavelength conversion of a wavelength converter such asa scintillator into consideration.

More specifically, according to the presentinvention, in a radiation detecting device or methodused in photographing using radiation emitted at aprescribed pulse duration, the detecting devicecomprises a wavelength converter for subjecting theradiation to wavelength conversion (for example, ascintillator which converts the radiation into aradiant ray having a wavelength in a visible region,and has a time constant of the afterglowcharacteristics of a luminescent screen) and aphotoelectrical conversion circuit part comprised ofpixels arranged in the form of a matrix and beingdriven by turning on the thin film transistors (TFTsfor transfer) every at least one column prescribed, which pixel is comprised of a thin film sensor elementhaving a prescribed capacity and a thin film transistor(TFT for transfer) having a prescribed ON resistanceand connected to the respective thin film sensorelements; and the radiation detecting method uses theserespective parts, wherein the detecting device ormethod is adapted to turn on the thin film transistorsafter the delay of at least (n × τ₁) [τ₁ being a timeconstant of the characteristics of the wavelengthconverter (the afterglow characteristics of aluminescent screen of the scintillator)], after theirradiation is stopped, thereby transferring signalsstored in their corresponding thin film sensorelements, the above objects can be achieved bydesigning a system so as to satisfy the followingrelationship:n = ln(SN);n × τ1 = ln(SN) × τ1wherein SN is a signal to noise ratio required of thesystem.

According to the present invention, in aradiation detecting device or method used inradiophotographing, radiation diagnostic apparatus orradiation therapeutic apparatus using radiation, thedetecting device comprises a wavelength converter forsubjecting the radiation to wavelength conversion (forexample, a scintillator which converts the radiation into a radiant ray having a wavelength in a visibleregion, and has a time constant of the afterglowcharacteristics of a luminescent screen); and aphotoelectrical conversion circuit part comprised ofpixels arranged in the form of a matrix and beingdriven by turning on the thin film transistors every atleast one column prescribed, which pixel is comprisedof a thin film sensor element having a prescribedcapacity and a thin film transistor having a prescribedON resistance and connected to the respective thin filmsensor elements; and the radiation detecting methoduses these respective parts, a desired signal to noiseratio (SN) required of the system can be obtained bysatisfying the following relational expressions:(α × τ1 + β × τ2) ≤ 1/FPS;SN = exp(α + β);and1/SN = exp(-α-β)wherein τ₁ is a time constant of the characteristics ofthe wavelength converter (the build up and attenuationof light emission of the scintillator); τ₂ is a timeconstant obtained by multiplying the capacity C of thethin film sensor element by the ON resistance R of thethin film transistor; FPS is the number of frames persecond upon the reading of the radiation detectingdevice, or a reciprocal of the time required for areading; a is a multiple of [(storage time of the light signal in the sensor)/(the time constant τ₁ of build upand attenuation of light emission of thescintillator)], or ln(SN₁) in which SN₁ is a signal tonoise ratio required of the wavelength converter(scintillator); and β is a multiple of a time constantof the time the thin film transistor (TFT) is turnedon, or ln(SN₂) in which SN₂ is a signal to noise ratiorequired of the TFT for transferring the signal storedin the capacitor of the thin film sensor element,whereby the above objects can be achieved.

According to the present invention, as describedabove, signals of a desired S/N ratio, which arereduced in noise and narrowed in scattering, can beread out by adopting the reading method taking thecharacteristics of the wavelength converter such as theattenuation characteristics of a scintillator intoconsideration, which enables us to reduce an exposeddose by intermittent exposure to radiation, or X-rays.

In addition, a radiation detecting device havingthe desired S/N ratio can be designed with ease byderiving the relationship for obtaining an optimumsignal to noise (S/N) ratio taking the attenuationcharacteristics of a scintillator into consideration inthe inspection, diagnosis and therapy with radiationcontinuously emitted.

According to the present invention, for example,a radiation detecting device comprising a large screen sensor panel with a plurality of thin film transistorsand thin film sensors having a-Si (amorphous silicon)two-dimensionally arrayed in the form of a matrix on aninsulating substrate, and a wavelength converter suchas scintillator arranged on the surface of the largescreen sensor panel can be easily designed as aradiation detecting device having the desired S/N ratioby relating a time constant τ obtained by multiplyingthe capacity C of the sensor by the ON resistance R ofthe thin film transistor, the read rate, S/N, and theattenuation characteristics of the wavelength convertersuch as the scintillator to the timing of irradiation.

The present invention will hereinafter bedescribed in details with reference to the drawings.

[First embodiment]

Fig. 2 is a schematic circuit diagram centeringon a radiation detecting device 100 for describing thisembodiment. In Fig. 2, a relationship between asubject 13 and a scintillator 14 or the like isschematically illustrated.

As illustrated in Fig. 2, the radiation detectingdevice according to this embodiment has a scintillator14 as a wavelength converter which converts radiation12 into photoelectrically convertible light, and pixels109 arranged in the form of a matrix which pixelscomprises sensor elements S1-1 to S3-3 for convertingthe light into an electric signal, a unit of which sensor element is represented by numeral 108 in Fig. 2,and TFTs (thin film transistors) T1-1 to T3-3 fortransfer, which are connected to the respective pixelsto successively transfer signals from the pixels.

In this embodiment, in order to obtain a desiredsignal to noise ratio SN, the detecting device furthercomprises a means (a shift resistor 102 in Fig. 2) toturn on the TFT to be turned on first among the TFTsT1-1 to T3-3 for transfer after causing delay by means(for example, a control circuit 15, a CPU 16 and aprogram memory 17 in Fig. 2) for delaying for at least(n × τ₁), wherein τ₁ is a time constant of the afterglowcharacteristics of a luminescent screen of thescintillator 14, and n is ln(SN), after the irradiationwith the radiation 12 is stopped, thereby transferringa signal stored in its corresponding pixel.

The radiation detecting device 100 comprises thescintillator 14 which converts the radiation 12 intovisible light, a photoelectric conversion part 101 inwhich pixels comprised of the thin film sensor elementsS1-1 to S3-3 having a-Si as a semiconductor layer forreceiving the visible light and converting into anelectric signal and the thin film transistors (TFTs fortransfer) T1-1 to T3-3 having a-Si as a semiconductorlayer for transferring the signal chargesphotoelectrically converted by the thin film sensorelements S1-1 to S3-3 on the side of matrix signal wirings M1 to M3 are two-dimensionally arrayed in theform of a matrix, and a shift resistor 102 for drivingthe gate lines G1 to G3 of the thin film transistor T1-1to T3-3. In this embodiment, the pixels areillustrated in a 3 x 3 matrix for the sake of briefdescription.

A capacitance 3 times as much as theinterelectrode capacitance (Cgs) of the thin filmtransistor is added to the matrix signal wiring M1 upontransfer. However, in Fig. 2, it is not indicated as acapacitor element. The same shall apply to the othermatrix signal wirings M2 and M3. The photoelectricconversion circuit part 101 in Fig. 2 comprised thethin film sensor elements (hereinafter also referred toas "photoelectric conversion elements") S1-1 to S3-3,the thin film transistor (hereinafter also referred toas "switching elements") T1-1 to T3-3, the gate drivingwirings G1 to G3 and the matrix signal wirings M1 toM3. These can be arranged on an insulating substratenot illustrated. The shift resistor (SR1) 102 servesas a driving circuit part for switching on or off theswitching elements T1-1 to T3-3.

Reference characters L1 to L3 indicateoperational amplifiers for amplifying and impedance-convertingthe signal charges from the matrix signalwirings M1 to M3. In Fig. 2, they are illustrated asbuffer amplifiers forming a voltage follower circuit. Reference characters Sn1 to Sn3 designate transferswitches for reading out the outputs from theoperational amplifiers L1 to L3, i.e., the outputs fromthe respective matrix signal wiring M1 to M3 andtransferring them to capacitors CL1 to CL3. The readcapacitors CL1 to CL3 are read out by the readingswitches Sr1 to Sr3 through buffer amplifiers B1 to B3forming a voltage follower circuit.

Reference numeral 103 designates a shift resistor(SR2) for switching on or off the reading switches Sr1to Sr3. The parallel signals from the read capacitorCL1 to CL3 are converted into a serial signal by thereading switches Sr1 to Sr3 and the shift resistor(SR2) 103, inputted into an operational amplifier 104comprising a final voltage follower circuit and furtherdigitized in an A/D converting circuit part 105.Reference characters RES1 to RES3 indicate resettingswitches for resetting the signal components stored inthe respective capacitors (3 Cgs) added to the matrixsignal wirings M1 to M3, and the signal components arereset to a desired reset potential (reset to a groundpotential of GND in Fig. 2) by a pulse from a CRESterminal.

Reference numeral 106 indicates a power sourcefor applying a bias to the photoelectric conversionelements S1-1 to S3-3. A reading circuit part 107comprises the buffer amplifiers L1 to L3, the transfer switches Sn1 to Sn3, the read capacitors CL1 to CL3,the buffer amplifiers B1 to B3, the reading switchesSr1 to Sr3, the shift resistor SR2, the finaloperational amplifier 104, and the resetting switchesRES1 to RES3. In the figure, a symbol "SMPL" denotes aSMPL terminal for SMPL pulses.

Fig. 3A is a schematic plan view illustrating anexample of a photoelectric conversion circuit part inwhich photoelectric conversion elements and switchingelements are fabricated using a thin amorphous siliconsemiconductor film 312. Fig. 3B is a schematic cross-sectionalview taken on line 3B-3B of Fig. 3A. Thinfilm sensor elements 301 and thin film transistors(amorphous silicon TFT; hereinafter referred to as"TFT" merely) 302 are formed in the same glasssubstrate 303. The lower electrode of each thin filmsensor element 301 and the lower electrode (gateelectrode) of each TFT 302 are formed by the same firstthin metal layer 304. The upper electrodes 305, 309 ofthe thin sensor elements 301 and the upper electrodes(source and drain electrodes) of the TFTs 302 areformed by the same second thin metal layer. The firstand second thin metal layers also form gate drivingwirings 306 and matrix signal wirings 307 in thephotoelectric conversion circuit part. In Fig. 3A, 4pixels in total of 2 x 2 are illustrated. In Fig. 3A,a hatched area indicates a light-receiving face of the thin film sensor element. The upper electrodes 305,309 are power supply lines for applying a bias to therespective thin film sensor elements. Referencenumeral 310 indicates a contact hole for connecting thethin film sensor element 301 to the TFT 302.

The thin film sensor element 301 has the same MISstructure in section as the TFT 302. The insulatingfilms 311 of the thin sensor element 301 and the TFT302 are composed of an insulating film formed incommon. A crossing area 314 of the gate driving wiring306 and the matrix signal wiring 307 has an insulatingfilm 311, a thin amorphous silicon semiconductor film312 and an ohmic conduct layer (n⁺ layer) 313 betweentheir wirings. Numeral 315 denotes an insulating layerof, for example, silicon nitride (SiN) film as aprotecting film, which is formed after forming thinfilm sensor element 301 and TFT 302.

Fig. 4 is an equivalent circuit diagram to thephotoelectric conversion circuit part in Fig. 2. Apixel comprised of the thin film sensor element and TFTis indicated by a square for the sake of convenience.

A bias line through which a bias is applied tothe respective thin film sensor elements is dividedinto 4 systems (Vs1 to Vs4), and the reset of thesensor can be conducted separately in the 4 systems.

Fig. 4 illustrates an example where the pixelsare arrayed in an n × m matrix. Since the sensor bias is divided into 4 systems, the number m of columns is amultiple of 4.

The operation of the radiation detecting deviceaccording to the first embodiment will now bedescribed. Fig. 5 is a timing chart illustrating anexample of the operation of the radiation detectingdevice as shown in Fig. 2 upon radiophotographing. Theoperation will be described in detail with reference tothe drawing.

Charges remaining in the wirings M1, M2 and M3after irradiation only for the time T by an X-raysource 11 are removed by turning on the CRES terminaland the transistors RES1 to RES3, thereby rendering thewirings M1, M2, M3 a ground potential.

The X-ray 12 emitted from the X-ray source 11 andtransmitted by subject 13 such as a structure or humanbody enters the scintillator 14, thereby causing thescintillator to emit light according to the quantity ofthe X-ray transmitted.

The light emitted from the scintillator 14 entersthe respective photoelectric conversion elements S1-1,S1-2, ... to S3-3 in the radiation detecting device100, and signal charges according to the quantity ofthe light incident on the respective photoelectricconversion elements S1-1 to S3-3 are generated.

The signal charges are stored only for a certainperiod of time in capacitor components formed in the photoelectric conversion elements S1-1 to S3-3. Thesignal charges stored in the photoelectric conversionelements S1-1 to S1-3 of the first line are transferredto capacitor components (capacitance 3 times as much asthe Cgs of the switching elements T1-1 to T3-3)respectively formed in the matrix signal wirings M1 toM3 by turning on the switching elements T1-1 to T1-3only for the time t1 according to a gate pulse signalG1 from the shift resistor (SR1) 102. In Fig. 5, M1 toM3 indicate the transfer where the quantities of thesignals stored in the respective photoelectricconversion elements vary. More specifically, in thephotoelectric conversion elements S1-1 to S1-3 of thefirst line, the output level is as follows:S1-2 > S1-1 > S1-3. The signal outputs from the matrixsignal wirings M1 to M3 are amplified by theoperational amplifiers L1 to L3, respectively.

Thereafter, the switching elements Sn1 to Sn3within the reading circuit part are turned on only forthe time t2 according to an SMPL pulse illustrated inFig. 5, whereby the signals are transferred to the readcapacitor CL1 to CL3, respectively. The signals in theread capacitor CL1 to CL3 are impedance-converted bythe buffer amplifiers B1 to B3, respectively.Thereafter, the reading switches Sr1 to Sr3 aresuccessively turned on according to shift pulses Sp1 toSp3 from the shift resistor (SR2) 103, whereby the parallel signal charges transferred to the readcapacitors CL1 to CL3 are converted into a serialsignal and read out. Assuming that pulse widths of theshift pulses Sp1, Sp2 and Sp3 are equal to one anotherand t3 (i.e., Sp1 = Sp2 = Sp3 = t3), the time requiredfor the serial conversion and reading amounts tot3 x 3. The signal serially converted is outputtedfrom the final operational amplifier 104 and furtherdigitized by the A/D converting circuit part 105.

Vout illustrated in Fig. 5 indicates an analogsignal before being inputted into the A/D convertingcircuit part 105. As illustrated in Fig. 5, theparallel signals from the S1-1 to S1-3 of the firstline, i.e., the parallel signals of the signalpotentials of the matrix signal wirings M1 to M3 areserially converted on the Vout signal in proportion tothe levels thereof.

Finally, the signal potentials of the matrixsignal wirings M1 to M3 are reset to a certain resetpotential (ground potential) through the respectiveresetting switches RES1 to RES3 by turning on the CRESterminal only for the time t4 to apply a CRES pulse,thereby providing for the next transfer of signalcharges from the photoelectric conversion elements S2-1to S2-3 of the second line. After this, thephotoelectrically converted signals of the second andthird lines are read out repeatedly in the same manner as in the first line.

At this time, the sensors store signals until thegate voltages (G1 to G3) of the TFTs are turned on.Accordingly, there is a deviation between the time thatG1 first turned on for transferring a sensor signal isturned on and the time that G3 last turned on is turnedon, so that the influence of the attenuation of lightemission of the scintillator varies every line. Thiswill be described with reference to Fig. 6. Fig. 6diagrammatically illustrates how sensor output variesafter stopping the irradiation.

In Fig. 6, an attenuation component is regardedas a signal component S as illustrated in the drawing,since the attenuation component is considered to bestored as a signal component in an area in which thesensor output is attenuated on an axis of ordinate.For example, when the TFTs for transfer are turned onupon the elapsed time of Tml after turning off theradiation to read out the signal charges stored, an S'component already stored can be read out as a signal.However, an N' component generated on and after the Tmlis left as a remaining component which may not betransferred, because it is not yet stored.Accordingly, this remaining component may be said to beanother noise component N' than the signal component.

Namely, the sensors of a line that the gates ofthe TFTs for transfer are earlier turned on after turning off the radiation start transfer in a shorterstorage time after stopping the irradiation.Therefore, their signal to noise ratio (S'/N') is low.On the other hand, the signal to noise ratio (S/N) ofthe sensors of a line that the gate is turned on later(for example, at Tm2) is high. Therefore, the signalto noise ratio varies with the line, resulting in thefact that the signal to noise ratio as the system isreduced.

However, a signal to noise ratio (SN) required asa system can be obtained by presetting the timecorresponding to the signal to noise ratio (SN)required as the system to the time constant τ₁ ofattenuation of the scintillator up to the time the gatevoltage of the TFTs for transfer is turned on.

Assuming that the quantity of light emitted bythe scintillator right before stopping the irradiationis 1, the quantity of light emitted by the scintillatorupon the elapsed time of n × τ₁ after stopping theirradiation amounts to exp(-n × τ₁). Accordingly, 1/SN= exp(-n) and ln(SN) = n can be set for the SN requiredas the system.

Accordingly, the desired SN can be obtained bypresetting the time until the TFTs for transfer arefirst turned on for reading out a sensor signal afterstopping the irradiation to Tm2 = n × τ₁ or longer asillustrated in Fig. 6.

The means in this embodiment for causing delay ofat least n × τ₁ will be now described in brief.

As means for controlling the delay time until theTFTs for transfer are first turned on to at leastn × τ₁, there are means in which a control program isdriven by, for example, a microcomputer (CPU) to startcounting at the time the irradiation by the radiationsource 11 is completed, taking the circuit illustratedin Fig. 2 as an example, and the G1 output of the shiftresistor SR1 is driven after delay of at least n × τ₁,and the like. Thus, this control can be easily carriedout by the conventional techniques.

It may also be easily carried out by providing asynchronizing signal line and a delay circuit betweenthe shift resistor SR1 and the radiation source 11 todelay a completion signal of the irradiation from theradiation source 11 in the delay circuit and then inputit as a start signal for the shift resistor.

[Second embodiment]

In this embodiment, an example where a desired SNis obtained in the case where many frame images arecontinuously read out to form a moving image will bedescribed.

Although the pixels of the radiation detectingdevice in the photoelectric conversion circuit part inFig. 2 are arranged in a 3 × 3 matrix, a case wherepixels are formed in an m (line) × n (column) matrix will be described in this embodiment. In this case, itis general to read out a sensor array at 30 frames persecond. At this time, a scanning time per frame is1/30 (sec), i.e., 33 msec.

Figs. 7A to 7E are timing charts illustrating anexample of timing upon reading out a moving radiationimage. As illustrated in Fig. 7A, the irradiation iscontinuously conducted.

Fig. 7B illustrates the case where reading orstorage of signals is continuously conducted.

Fig. 7C illustrates the case where after thewhole array is read out, a quiescent time Q is providedbefore the next reading is started (indicated by abroken line in the drawing). The quiescent time can bepreset to at most t_f - t_x•q wherein t_f is a scanningtime per frame, t_x is a read time per column, and q isthe number of columns read (≤ n).

Fig. 7D illustrates the case where each column isread out at a certain timing, but a quiescent time isprovided between reading times of the respectivecolumns (while storage is being conducted).

Fig. 7E illustrates the case where a quiescenttime is provided between reading times of therespective columns like the case in Fig. 7D. However,during this quiescent time, storage is also notconducted in each column. An S/N ratio can be furtherenhanced by, for example, conducting (refreshing) drive that a remaining component after discharged (charged)from the sensor is removed.

Essential parameters relating to reading in asystem including a radiation detector include thefollowing four parameters:

(1) SN required of the system;

(2) CR time constant τ₂ according to a sensor anda switch in each pixel;

(3) scanning speed (the number of frames)required of the device; and

(4) time constant τ₁ of light emission of ascintillator as to build up upon exposure to an X-rayand attenuation after the exposure to the X-ray.

The parameters (1) to (4) will be described inmore detail.

(1) The SN of a signal from a sensor panel isdefined by the quantity S of a signal transferredthrough a switch and the quantity N of a signal leftafter the transfer.

(2) The CR time constant τ₂ is a value obtained bymultiplying a storage capacity C of the sensor by an ONresistance R of the switch (TFT) in each pixel.

(3) The scanning speed (the number of frames) isthe number of scanning operations (the number offrames) of n columns (q < n columns as needed) persecond. In the normal monitor, it is 30 frames/sec.

(4) The build up of light emission of a scintillator upon exposure to the radiation and theattenuation of light emission of the scintillator afterthe exposure to the radiation show a multiple-exponential(Σαtⁿ) change. However, it is defined asbeing expressed by an exponential function according tothe time constant τ₁ in the present invention.

Fig. 8 diagrammatically illustrates an example ofthe relationship between the transfer time of TFT, andthe quantities of signals transferred and signals leftafter the transfer, and shows the transferred quantityin the case where signal charges stored in capacitorswithin the photoelectric conversion elements (S1-1 toS3-3 in Fig. 2 by way of example) are defined as 1.Consideration is given as to the above parameters withreference to Fig. 8.

When transfer is conducted β times as much as thetime constant τ₂ (wherein β = t/τ₂), the transferredquantity of the signal component S may be expressed byS = 1 - exp(-β) as illustrated in Fig. 8 when theparameter (1) is combined with the parameter (2). Thecomponent N₂ left after the transfer is expressed byN₂ = exp(-β) as illustrated in Fig. 8. When thetransferred signal component is defined as S, S comesto 1 - N₂ = 1 - exp(-β). Since exp(-β) « 1, S may besaid to be nearly equal to 1 when standardized.

On the other hand, since SN₂ = S/N₂, SN₂ may be expressed by S/N₂ = 1/exp(-β). Accordingly, thereciprocal of SN₂ comes to 1/SN₂ = exp(-β) = N₂.Namely, the component N₂ left after the transfer becomesa reciprocal of SN₂, or β is equal to lnSN₂.

The scattering of output based on the parameter(4) will now be considered with reference to Fig. 6.

1) The output S of a scintillator having a timeconstant τ₁ of light emission after the time t fromlight emission according to the build up of the lightemission is expressed by S = S₀•(1 - exp(-α)) wherein S₀is a sensor output when saturated, and α is t/τ₁.

2) The change in output due to the delay ofattenuation of light emission of the scintillator isexpressed by S = S₀•exp(-α).

When the time constant of build up andattenuation of the scintillator is 0, namely, a changeis caused momentarily, there is no noise. However,since the scintillator has the time constant τ₁, a noisecomponent occurs in a proportion of exp(-α). At thistime, the output S may be said to be nearly equal to 1.In actual reading, the storage time is almost areciprocal of the number of frames. When the number offrames is 30 frames/sec by way of example, the storagetime is 33 msec. Therefore, good approximation isachieved when the time constant of the scintillator ison the order of milliseconds.

Namely, 1/SN₁ = exp(-α) is satisfied for SN₁required.

SN required from the outside as the systembecomes a synthetic SN of SN₂ = exp(β) come about fromthe time constant of the TFT and SN₁ = exp(α) come aboutfrom the time constant of the scintillator.

A reciprocal of this synthetic SN can be set to1/SN = exp(-α-β).

The time of the TFT for transferring a sensorsignal is β × τ₂, and the time during which light fromthe scintillator is received by the sensor to store asignal therein is α × τ₁. The total time of α × τ₁ andβ × τ₂ cannot exceed the time of 1 frame.

Accordingly, the following relational expressionis satisfied.(α × τ1 + β × τ2) ≤ 1/FPSwherein FPS is the number of frames per second upon thereading of the radiation sensor; τ₁ is a time constantof build up and attenuation of light emission upon andafter the irradiation of the scintillator with theradiation; τ₂ is a time constant obtained by multiplyinga sensor capacity by the ON resistance of the TFT; α isa multiple of [(storage time of the light signal in thesensor)/(the time constant of build up and attenuationof light emission of the scintillator]; and β is a multiple of a time constant of the time the TFT fortransfer is turned on.

Accordingly, when the scanning time of the sensorper frame is preset to at least (α × τ₁ + β × τ₂) by, forexample, a control system using a microcomputer, aradiation detecting device having a desired signal tonoise ratio, SN = ln(α + β), can be obtained with ease.Incidentally, assuming that a signal to noise ratiorequired of the scintillator is SN₁, α may berepresented by ln(SN₁), while β may be represented byln(SN₂) in which SN₂ is a signal to noise ratio requiredof the TFT for transferring the signal stored in thecapacitor of the sensor element.

As described above, when the relationship betweennecessary SN and read rate is preset to the optimumalso in systems of radiophotographic apparatus,radiation diagnostic apparatus and radiationtherapeutic apparatus, various radiation detectingdevices having a satisfactory signal to noise ratiotaking the time constant of afterglow characteristicsof a phosphor and the time constant come about from thethin film sensor element and the thin film transistorinto consideration can be provided.

According to the present invention, radiationdetecting methods and apparatus capable of conductingstable reading can also be provided.

Further, according to the present invention, radiation detecting devices and methods can be providedat lower cost because the design can be made with easeaccording to performance requirements.

In the present invention, the radiation is notlimited to X-rays, and α-rays, β-rays, γ-rays and thelike may also be applied to the systems in whichinformation subjected to wavelength conversion by awavelength converter is outputted in the form of anelectric signal by a photoelectric conversion element.However, it is desirable to apply the present inventionto systems using X-rays, which are widely in commonuse. As the wavelength converter, a scintillator (orphosphor) having a time constant in wavelengthconversion characteristics by the incidence of lightfrom a light source is preferably used.

Naturally, the present invention may be suitablymodified within the scope and sprit of the appendedclaims.

Claims (31)

1. A radiation detecting device having awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, thedetecting device comprising:
      a means for turning on the TFT to be turned onfirst among the TFTs for transfer after the delay of atleast (n × τ₁), wherein τ₁ is a time constant of acharacteristic of the wavelength converter, and n isln(SN) in which SN is a desired signal to noise ratio,after the irradiation is stopped, thereby transferringa signal stored in its corresponding pixel.
2. The radiation detecting device according toClaim 1, which further comprises a radiation source foremitting the radiation.
3. The radiation detecting device according toClaim 2, wherein the radiation source emits radiationselected from the group consisting of α-rays, β-rays, γ-raysand X-rays.
4. The radiation detecting device according toClaim 1, wherein the wavelength converter comprises ascintillator.
5. The radiation detecting device according toClaim 4, wherein the scintillator comprises a phosphor.
6. The radiation detecting device according toClaim 1, wherein the wavelength converter is ascintillator, and the characteristic of the wavelengthconverter is an afterglow characteristic of thescintillator.
7. The radiation detecting device according toClaim 6, wherein the afterglow characteristic is anattenuation characteristic of the scintillator.
8. A radiation detecting device comprising awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, whereinthe detecting device satisfies the following relationalexpressions:(α × τ1 + β × τ2) ≤ 1/FPS;andSN = exp(α + β)wherein SN is the desired signal to noise ratio of thewhole device; FPS is the number of frames per secondupon the reading of the radiation detecting device, ora reciprocal of the time required for a reading; τ₁ is atime constant of build up and attenuation of thewavelength converter; τ₂ is a time constant obtained bymultiplying the capacity of the sensor element by theON resistance of the TFT for transfer; α is a multipleof [(storage time of the light signal in the sensorelement)/τ₁], or ln(SN₁) in which SN₁ is a signal tonoise ratio required of the wavelength converter; and βis a multiple of a time constant of the time the TFTfor transfer is turned on, or ln(SN₂) in which SN₂ is asignal to noise ratio required of the TFT fortransferring the signal stored in the capacitor of thesensor element.
9. The radiation detecting device according toClaim 8, which further comprises a radiation source foremitting the radiation.
10. The radiation detecting device according toClaim 9, wherein the radiation source emits radiationselected from the group consisting of α-rays, β-rays, γ-rays and X-rays.
11. The radiation detecting device according toClaim 8, wherein the wavelength converter comprises ascintillator.
12. The radiation detecting device according toClaim 11, wherein the scintillator comprises aphosphor.
13. The radiation detecting device according toClaim 8, wherein the wavelength converter is ascintillator, and the time constant of build up andattenuation of the wavelength converter is the timeconstant of build up and attenuation of thescintillator.
14. The radiation detecting device according toClaim 13, wherein the time constant of build up oflight emission of the scintillator is 1.
15. A radiation detecting method using awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor (TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, themethod comprising:
       turning on the TFT to be turned on first amongthe TFTs for transfer after the delay of at least(n × τ₁), wherein τ₁ is a time constant of acharacteristic of the wavelength converter, and n isln(SN) in which SN is a desired signal to noise ratio,after the irradiation is stopped, thereby transferringa signal stored in its corresponding pixel.
16. The radiation detecting method according toClaim 15, which further uses a radiation source foremitting the radiation.
17. The radiation detecting method according toClaim 16, wherein the radiation source emits radiationselected from the group consisting of α-rays, β-rays, γ-raysand X-rays.
18. The radiation detecting method according toClaim 15, wherein the wavelength converter comprises ascintillator.
19. The radiation detecting method according toClaim 18, wherein the scintillator comprises a phosphor.
20. The radiation detecting method according toClaim 15, wherein the wavelength converter is ascintillator, and the characteristic of the wavelengthconverter is an afterglow characteristic of thescintillator.
21. The radiation detecting method according toClaim 20, wherein the afterglow characteristic is anattenuation characteristic of the scintillator.
22. A radiation detecting method using awavelength converter for converting radiation intophotoelectrically convertible light and a plurality ofpixels arranged in the form of a matrix which pixelcomprises a sensor element for converting the lightinto an electric signal and a thin film transistor(TFT) for transfer connected to the sensor element tosuccessively transfer a signal from the pixel, themethod comprising satisfying the following relationalexpressions:(α × τ1 + β × τ2) ≤ 1/FPS;andSN = exp(α + β)wherein SN is the desired signal to noise ratio of thewhole device; FPS is the number of frames per secondupon the reading of a radiation detecting device, or a reciprocal of the time required for a reading; τ₁ is atime constant of build up and attenuation of thewavelength converter; τ₂ is a time constant obtained bymultiplying the capacity of the sensor element by theON resistance of the TFT for transfer; α is a multipleof [(storage time of the light signal in the sensorelement)/τ₁], or ln(SN₁) in which SN₁ is a signal tonoise ratio required of the wavelength converter; and βis a multiple of a time constant of the time the TFTfor transfer is turned on, or ln(SN₂) in which SN₂ is asignal to noise ratio required of the TFT fortransferring the signal stored in the capacitor of thesensor element.
23. The radiation detecting method according toClaim 22, which further uses a radiation source foremitting the radiation.
24. The radiation detecting method according toClaim 23, wherein the radiation source emits radiationselected from the group consisting of α-rays, β-rays, γ-raysand X-rays.
25. The radiation detecting method according toClaim 22, wherein the wavelength converter comprises ascintillator.
26. The radiation detecting method according toClaim 25, wherein the scintillator comprises a phosphor.
27. The radiation detecting method according toClaim 22, wherein the wavelength converter is ascintillator, and the time constant of build up andattenuation of the wavelength converter is the timeconstant of build up and attenuation of the scintillator.
28. The radiation detecting method according toClaim 27, wherein the time constant of build up of lightemission of the scintillator is 1.
29. A timing control unit for use in the radiationdetecting device of Claim 1, which unit constitutes themeans for turning on the TFT to be turned on first amongthe TFTs for transfer after the delay of at least (n xτ₁), wherein τ₁ is a time constant of a characteristic ofthe wavelength converter, and n is ln(SN) in which SN isa desired signal to noise ratio, after the irradiationis stopped, thereby transferring a signal stored in itscorresponding pixel.
30. A timing control unit according to Claim 29comprising a control circuit, a central processing unitco-operative with said control circuit, and a programmemory in combination therewith, wherein said program memory contains executable instruction code for operatingsaid control circuit to perform the function of turningon the TFT to be turned on first in the manner aforesaid.
31. A program memory for use in the timing controlunit of Claim 30, wherein said program memory containsexecutable instruction code for operating said controlcircuit to perform the function of turning on the TFT tobe turned on first in the manner aforesaid.

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